The invention relates to an article that includes a silicon-containing substrate and an intermediate layer/coating with an external barrier coating such as a protective environmental/thermal barrier coating (E/TBC).
Silicon-containing substrates have been proposed for structures used in high temperature applications, such as in heat exchanges and advanced internal combustion engines. Silicon-containing substrates are also used in gas turbine engines. Higher operating temperatures increase the efficiency of gas turbine engines. Silicon-based composite ceramics have been proposed as materials for applications in combustors for supersonic commercial airplanes. However, in many applications involving water-containing environments, a silicon-based substrate will recede and lose mass because of the formation of volatile species, such as silicon hydroxide [Si(OH).sub.4 ]. The recession rate due to the volatilization or corrosion is often unacceptably high so that an external barrier coating such as an environment/thermal barrier coating (E/TBC) with high resistance to such environments is required.
The external barrier coating can be an environmental/thermal barrier coating (E/TBC) that comprises a chemically stabilized zirconia, such as yttria stabilized zirconia. These coatings are capable of preventing the substrate materials from being in direct contact with environmental oxygen that diffuses through the coatings fairly rapidly and reaches the underlying silicon-containing substrate. Oxidation of the silicon-containing substrate involves the formation of various gaseous products. For example, the following equations demonstrate the attack on silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4): EQU SiC(s)+O.sub.2 (g).fwdarw.SiO.sub.2 (S)+CO.sub.x (g) (x=1,2) EQU Si.sub.3 N.sub.4 (s)+O.sub.2 (g).fwdarw.SiO.sub.2 (S)+NO.sub.x (g) (x=0.5-3)
The form of the gaseous products is dependent on the oxygen partial pressure in the system. These gaseous species have low solubility and diffusivity in silica (SiO.sub.2) and in other oxides, which causes them to be trapped at the external coating/substrate interface to form voids. The pressure of the gases in the voids can be sufficiently high at elevated temperatures to cause bursting. Voids can also interconnect to form large unbounded interfacial regions that result in coating spallation.
Thus, there is a need to prevent formation of gaseous oxidation products at an interface region between an environment/thermal barrier coating (E/TBC) and a silicon-based substrate.